The present disclosure relates to a method and to a device for application of a pressure to a battery, which comprises at least one or more cells, in order to reduce adverse effects on operation which occur because of different states of battery charge.
Batteries for storing electrical energy, especially lithium ion batteries, expand as they are charged and contract again as they are discharged. These changes in volume and length are caused by the absorption and release processes of lithium ions in the active materials of the electrodes. The absorption of lithium into the carbon material causes the material to expand. This increase in volume is transmitted outward via the case of the battery cell, provided the cell has a corresponding capacity for expansion, and thus leads to a change in at least one geometric dimension of the battery; especially in situations in which the battery comprises a plurality of cells.
Where the battery is embodied with a substantially rigid housing, the expansion of the carbon material causes a buildup of the pressure-induced stress in the respective battery cell. This can have the effect of exposing the individual layers arranged in the cell (metal layer, cathode material, separator, anode material, film where applicable) to mechanical stresses due to the expansion and contraction of the electrode materials. The result is a rise in the electrical resistance in the battery and hence reduced performance.
One known way of preventing the mechanical stress described is to apply a constant contact pressure, as illustrated in FIG. 1. FIG. 1 shows a battery 10 having a plurality of cells 11, the cells 11 being arranged in a pack or a stack. Constant pressure forces 16 are introduced into the cells 11 by means of flanges 12 arranged above and below the pack of cells. This results in reaction forces 17 prevailing between the cells. The constant contact pressure is intended to prevent the unwanted separation of individual layers from the electrode materials. Such a constant contact pressure is employed, in particular, in battery cells for hybrid vehicles. These cells are generally operated at a state of charge (SOC) of, on average, 30 to 70%, generally 50%. The lithium ion batteries provided for electric vehicles are operated at states of charge of from 0 to 100%. Batteries for electric vehicles are also of similar construction to those for hybrid vehicles.
Since the volume increases at higher states of charge, and the volume decreases at smaller states of charge, the battery cells of a battery for an electric vehicle are in some cases subjected to considerable expansions in volume and contractions in volume. This results from continuously alternating stresses due to discharges during acceleration of the vehicle and when charging during recuperative operation of the brakes.
However, especially in the case of more pronounced increases in volume and reductions in volume of the active materials within the cell, pressing the battery cells together firmly at a constant pressure (without a change in length of the stack of cells) may result in the separator being overcompressed, and this likewise results in an increase in the internal electrical resistance of the battery cell. In addition, anode material and possibly also cathode material may be elastically or plastically deformed, and this likewise leads to a rise in the internal electrical resistance.
Further mechanical stresses on a battery or a battery cell can arise, for example, from an increase in temperature within the cell and the resulting evaporation of electrolytes contained therein, the vapor leading to a further rise in pressure within the cell. Especially at elevated temperatures, chemical reactions in the battery or in a cell can furthermore occur, giving rise to gases which generate an additional rise in pressure within the battery cell.
These stresses are all the higher, the greater the fluctuations in the state of charge during the operation of the battery.
In the case of conventional batteries and especially lithium ion batteries, it is thus difficult, on the one hand, to avoid the mechanical stresses which can lead to damage to layers within a battery cell and, on the other hand, to ensure that the internal resistance in the battery or in a battery cell is not increased over its time in service.
WO 2006/112639A1 has disclosed the arrangement of piezoelectric sensors for detecting the internal pressure within a battery. By detecting when a particular predetermined internal pressure is exceeded, it is possible to initiate suitable countermeasures. However, it does not disclose what countermeasures are suitable.
It is therefore the underlying object of the disclosure to provide a method and a device by means of which premature battery wear or a premature battery aging process can be prevented and a sufficiently low internal resistance can be ensured in a simple manner.
The present object is achieved by the method as and by the device as set forth below.
Advantageous embodiments of the method are described below, and advantageous embodiments of the device according to the disclosure are also described below.
As a supplementary measure, a motor vehicle which has the device according to the disclosure is furthermore provided in accordance with the disclosure as set forth below.